These conclusions in themselves are important factors in the study of
expanding models of the universe,
but the implications of the empirical data can be pushed
still farther. The nature of the expansion and the
nature of the spatial curvature are now determined, but
there still remains a third arbitrary element in the
general theory, namely, the cosmological constant.
This constant occurs in formulae connecting both of
the other elements with the contents of the universe.
Now we know the nebulae well enough, but we do not
know what lies between the nebulae. Consequently,
we cannot directly determine the numerical value of
the cosmological constant. Nevertheless, with the help
of two presumably necessary assumptions, we can make
a reliable estimate of the order of the constant. The
assumptions are that neither the mean density in the
universe, nor the mean pressure, is less than zero -
that the universe is not less than empty, either of
matter or of radiation. Then, with the aid of the known
elements of expansion and curvature, we can set upper
and lower limits between which the constant must lie.
Fortunately, the limits are so close together that we
may state the approximate value of the constant with
confidence. It is about 4.5 × 10-18 (years)-2.

The significant features are the facts that the constant
is positive, and is slightly larger than a certain critical
value - the value it would have in a particular, unstable
model called the Einstein universe. These facts assign
the actual world to a single class known as monotonic,
or `ever-expanding', universes. The other elements are
sufficient to identify, within the class, a unique model
known as the ever-expanding universe of the first kind.

The model expands without reversal. The radius
increases from zero to infinity. Past time is finite,
future time is infinite. Comparatively recently, perhaps
a thousand million years ago, the model started to expand from a small
compact mass. The expansion was
very rapid at first, but it has steadily slowed down to
the rate we measure today. At each moment the
model is homogeneous, the contents are uniformly
distributed, but as time goes on the mean density
diminishes, the-average distance between neighbouring nebulae
increases. Eventually a state of complete isolation will be reached.

The disturbing features of this picture of the universe is the very
small scale both in time and in space.
With existing telescopes we explore at least a third of
the `age' and about a quarter of the volume. And there
is a third questionable feature. The general formulae
indicate a definite relation between spatial curvature,
the cosmological constant, and the contents of space.
The universe suggested by the surveys is very massive.
The mean density is of the order of a thousand times
that which can be accounted for by the nebulae we
observe. The excess matter, if it exists, must be scattered between the
luminous nebulae.

We suppose that such internebular material would
probably be in the form of dust or ionized gas. In that
case the medium would absorb light, and would be
readily detected. But we find no trace of space absorption; space is
sensibly transparent. Therefore, the
matter, if it exists, must be in some unlikely form such
as chunks or non-ionized gas. Even then, the necessary
quantity is barely permissible. For, on other evidence,
we can set an upper limit to the possible density, regardless of the
form of the material. We find no trace
of internebular material, but our investigations have
been pushed only to a certain limit. At the moment,
that limit is just about the density corresponding to the
curvature. Thus the theory might be valid provided
the universe were packed with matter to the very
threshold of perception. Nevertheless, the ever-expanding model of the
first kind seems rather dubious.
It cannot be ruled out by the observations, but it suggests a forced
interpretation of the data.

The disturbing features are all introduced by the recession factors, by
the assumption that red shifts are
velocity-shifts. The departure from a linear law of
red-shifts, the departure from uniform distribution, the
curvature necessary to restore homogeneity, the excess
material demanded by the curvature, each of these is
merely the recession factor in another form. These
elements identify a unique model among the array of
possible expanding worlds, and, in this model, the restriction in the
time-scale, the limitation of the spatial
dimensions, the amount of unobserved material, is each
equivalent to the recession factor.

On the other hand, if the recession factor is dropped,
if red-shifts are not primarily velocity-shifts, the picture
is simple and plausible. There is no evidence of expansion and no
restriction of the time-scale, no trace
of spatial curvature, and no limitation of spatial dimensions. Moreover,
there is no problem of internebular material. The observable region is
thoroughly homogeneous; it is too small a sample to indicate the
nature of the universe at large. The universe might
even be an expanding model, provided the rate of
expansion, which pure theory does not specify, is inappreciable. For
that matter, the universe might even be contracting.